THEORETICAL AND APPLIED MECHANICS TEORIJSKA I PRIMENJENA MEHANIKA  2009 Vol.36(1), pp.47-83 Summary: Djordje Musicki A contribution to the theory of the extended Lagrangian formalism for rheonomic systems In this paper the generalization of the notion of variation in the extended Lagrangian formalism for the rheonomic mechanical systems (Dj. Musicki, 2004) is formulated and analyzed in details. This formalism is based on the extension of a set of generalized coordinates by new quantities, which determine the position of the frame of reference to which the chosen generalized coordinates refer. In the process of varying, the notion of variation is extended so that these introduced quantities, being additional generalized coordinates, must also to be varied, since the position of each particle of this system is completely determined only by all these generalized coordinates. With the consistent utilization of this notion of variation, the main results of this extended Lagrangian formalism are systematically presented, with the emphasis on the corresponding energy laws, first examined by V. Vujicic (1987), where there are two types of the energy change laws and the corresponding conservation laws. Furthermore, the generalized Noether's theorem for the nonconservative systems with the associated Killing's equations (B. Vujanovic, 1978) is extended to this formulation of mechanics, and applied for obtaining the corresponding energy laws. It is demonstrated that these energy laws, which are more general and more natural than the usual ones, are in full accordance with the corresponding ones in the vector formulation of mechanics, if they are expressed in terms of quantities introduced in this extended Lagrangian formalism. Finally, the obtained results are illustrated by an example: the motion of a damped linear harmonious oscillator on an inclined plane, which moves along a horizontal axis, where it is demonstrated that there is valid an energy {like conservation law of Vujanovic's type.